Propane treating in cracking systems



Feb. 15, 1938. 7 ATwELL 2,108,636

PROPANE TREATING IN CRACKING SYSTEMS Filed Feb. 16, 1934 fibsor'ber 5 $5 Prelim;

7&1" Flash,

Clean Tar INVENTOR ATTORNEY atented Feb. 1, I13

namia v. Atwell, White Plains, n. y, a .to Standard Oil Company, Chicahh, a cor- I pcration of Indiana Application February 16, 1934 Serial at. that"! 4 Claims. (cl. 196-13) This invention relates to the use of propane in cracking systems and it pertains more particularly to an improved low temperature method of filtering pressure tar and of recovering and utilizing valuable products resulting from cracking operations.

In'the production of cracked gasoline, mineral oils or other carbonaceous materials are heated to about I" F. to 1250 F. at pressures ranging from atmospheric to 1000 pounds per square inch, and the reaction products are usually maintained ,under pressure at elevated temperatures for a.

sufficient period of ltime to allow the conversion of the oils or carbonaceous materials into light hydrocarbons such as gasoline, heavy hydrocarbons generally called pressure tar, and gases containing appreciable amounts of propane, ethane, propylene, ethylene, etc. It is conventional practice to use lime or equivalent materialv inv the cracking stock for preventing corrosion, for preventing carbon deposition or for other purposes. This lime or other solid material accumulates in the pressuretarv and it is conventlonal practice to filter the pressure tar at tem-- peratures of about 350-500 F. (above the flash point of the oil) to remove the lime, together with coke particles or other solids. The high temperature is necessary because of theextremely high viscosity of the tar at lower-temperatures. However high temperature pressure-tar filters require the use of special filter elements which commonly consist largely of asbestos.

An object of my invention is to provide a method and means of filtering pressure tar at low temperatures.

A further object of my invention is to utilize the I pressure tar as an absorbing oil for removing propane and other normally gaseous hydrocarbons from the gaseous products resulting from v the cracking operation-from the so-called high line gases or even the so-called wet gas."

A further object is to provide an improved cruising system which includes .the recovery and utilizationoi propane and other normally gaseous hydrocarbons.

Other objects will be apparent as the detailed description of my invention proceeds.

in practicing ,the invention I may crack carbonaceous materials such as gas oil or other charging'stock by subjecting it to elevated tem peratures and pressures in any conventional apparatus. The low-boilingproducts, including cycle stock, light and heavy naphtha, condensables and fixed gases, are fractionated, the cycle stock being returned to the heater oi the cracking unit,

the naphtha or gasoline being withdrawn for use, and the dry gas being scrubbed with and partially absorbed by the tar which has been previously cooled, preferably by heat exchange with incoming stock. The absorption of these gases in the cooled pressure tar accomplishes at least three important results: (1) it reduces the viscosity of the cooled tar so that the tar may be readily handled and filtered at ordinary room temperatures through simple canvas filter elements in conventional closed filters; (2) it facilitates a sharp separation between valuable tarry and asphaltic compounds and lime, coke and objectionable carbonaceous solids whereby increased yields of by-products are obtained and decreased yields of obnoxious, dimcultly dis-' posable residues are obtained; and (3) it provides a means for utilizing lime (which would otherwise be wasted) for sweetening the cracked gases and removing objectionable sulfur compounds therefrom, the sweetened gases being recovered in the auto-tar distillation process. Incident to the above accomplishments are increased yields of valuable products, increased filtration rates, decreased labor and equipment requirements and decreased loadon the treating units. Other advantagesi will be apparent from the following detailed description.

In the accompanying drawing which forms a part of this specification, l. have shown a diagrarrm'iatic layout of my improved system.

The feed stock, which may be a petroleum gas oil, for example 3ll35 A. P. I. midcontinent gas oil or any other carbonaceous material which can be converted into lighter or more valuable products by cracking or polymerization, is introduced through line it, reflux coil l2 and earchanger it to pipe still it in which it is heated to cracking or polymerizationtemperatures. The

hot miirture is then'introduced throughline it to soaking ,drum it which may be. an enlarged reaction vessel'or which may be merely an elongated conduit of small cross sectional area connected to a hash chamber for separating tars from vapors and gases. The vapors and, gases are conveyed by pipe ll to a lower or interme-' diate point of fractionatlng column 58, the bottoms" from this column being recycled through line l to the cracking or polymerization heater. If. desired, I may introduce the feed stock through valved line it directly into line ill and introduce it into the heater with the cycle stools in line 09.

Vaporized naphtha and normally gaseous. hydrocarbons are moved from the top of column l8 through line 2| and are passed through conthereby sweetened to a material extent.

denser 22 to separating chamber 23. The temperature and pressure conditions in the separating chamber may vary throughout a relatively wide range, it being designed to remove normally liquid products such as pentane, hexane, etc. from normally gaseous products suchas propane, ethane, butane, propylene, ethylene, etc. The liquids from separator 23 are introduced through line 24 to a stabilizer 25 which is provided with a suitable heater 28 and with reflux means 21. Gasoline is withdrawn through line 28 and the propane and normally gaseous products removed from'the gasoline are withdrawn through line 29 to storage tank 30 thence to lines 3| or 32,

which will be hereinafter described.

- The dry gas from the top of separator 23 is led through pipe 33 to a lower or intermediate point of absorption tower 34 wherein it is scrubbed by tar which has been withdrawn from soaking drum I 6 through hot tar line 35, cooled in cooler 36, and led to the top of the absorption tower by pipe 31 which should be short and of large cross sectional area. Methane and unabsorbed gases may be vented from the top of the absorber through pipe 38 which may lead into the conventional refinery dry gas line or high line. Likewise gas from the top of storage 30 may be vented through pipe 33a and propane from line 3| may be introduced into the tar scrubber with gases from line 33. The absorber is preferably operated under a pressure of from 250 to 400 pounds and at ordinary atmospheric temperatures of approximately to 100 F. Under these conditions a suflicient amount of propane is absorbed by the pressure tar to cause a strlk ing decrease in viscosity and to effect a thorough washing of the lime particles. These lime particles are thereby rendered accessible for chemical action on the undesirable sulfur compounds which are present in the gases and the propane introduced through pipes 3| and 33 and the gases which are introduced through these pipes are The amount of liquefied gases which are absorbed by the pressure tar is usually less than 10% of the volume of the tar and under these conditions there is no appreciable phase separation of asphaltic compounds from propane. .However, the reduction of viscosity is so great that filtration may be readily effected through ordinary canvas filter leaves either in batch filters or in continuous rotary filters. The lime, coke and carbonaceous particles separate readily from the diluted tar to'give a cake which may be easily discharged and which may be disposed of much more readily than hot filter cakes heretofore obtained. Thus it will be seen that I have eliminated the necessity of using asbestos filter elements and of operating at temperatures of from 400 to 500 F. and'at the same time I have made it possible to obtain very rapid filter rates and to obtain almost quantitative separation of valuable by-products. p

The relatively cool diluted pressure tar may be passed by line 39 to filter 40 or it may be passed to line 4| and settler 42 or it may be passed'first to settler 42 and the decanted liquids passed through pipe 43 to the filter. Tar in line 39 may be further diluted with propane from pipe 32.

The filtered or settled dilute pressure tar from which all solid matters have been removed is introduced by pipes 44 or 45 to flash drum 46, it

being understood that a suitable collector and feed pump maybe interposed in this line and that suitable heat exchangers may be employed to supply the heat necessary for removing the propane, naphtha and cycle stock from the flash drum residue which may be used as a fuel oil or as a road oil. Flash drum 46 corresponds to the conventional auto-tar plant.

Settled material from settler 42 may be withdrawn by pipe 41 and filtered material from filter 40 may be withdrawn through conduit 48, and in each case the residue may be flashed in chamber 49 to recover propane and valuable oils through line 50. Unevaporated liquids may be drawn from flash drum 49 by line 49a. Fuel oil, road oil or tar is withdrawn from fiash drum 46 through pipe 5| and the propane, together with naphtha vapors and heavier auto-tar distillates are removed through pipe 52. Propane may be passed by line 53 or compressed by pump 54 and then passed to condenser 55. Liquids are collected in tank 56, gases being withdrawn through vent 51 to a gas fuel line and sweetened propane and heavier liquids being withdrawn through line 58.

Instead of cooling the entire volume of pressure tar before absorbing propane vapors, I may flash the tar by introducing it through line 59 into auto-tar still 60 which may be supplied with steam heater 6| if the heat of the tar is not sufficient to effect the desired vaporization. Autotar distillate may be removed through line 62, condenser 63, receiver 64, etc., and the residue may be passed by line 65 to line 35 or line 31.

While I have described in detail the preferred embodiment of my invention, it should be'understood that I do not limit myself to the specific arrangement hereinabove set forth or to any of the details hereinabove described except as defined by the following claims which should be construed as broadly as the prior art will permit.

I claim:

1. In a process wherein hydrocarbon oil is sublected to conversion conditions of temperature and pressure to produce motor fuel, and a relatively viscous pressure tar which is characterized by having asphalt in solution and by having suspended contaminating insoluble solid matter therein which renders saidtar diiiicultto filterthe improvement in refining said pressure tar to make it suitable for fuel purposes which comprises cooling said tar, conducting said cooled tar into an absorption chamber, conducting normally gaseous the said tar and dissolved normally gaseous hy-- drocarbons into a separating zone and separating said contaminating insoluble solid matter from the said viscosity-reduced tar, and thereafter removing the dissolved gaseous hydrocarbons from the tar.

2. In a process wherein hydrocarbon oil is subjected'to conversion conditions of temperature and pressure to produce motor fuel, normally gaseous hydrocarbons substantially free from gasoline, and a relatively viscous pressure tar which is characterized by having asphalt in solution and by having suspended contaminating insoluble solid matter therein which renders said said pressure tar to make it suitable for fuel purposes whichcomprises cooling said tar, conducting said cooled tar into an absorption chamber, conducting said normally gaseous hydrocarbons to said chamber, intimately contacting said cooled tar and said normally gaseous Hydrocarbons and dissolving about one liquid volume of said normally gaseous hydrocarbons in each ten liquid volumes of said tar, thus decreasing the viscosity of said tar without precipitating any substantial amount of asphalt therefrom, separatingundissolved gaseous hydrocarbons from the viscosityreduced tar, passing the said tar and dissolved normally gaseous hydrocarbons into a separating zone and separating said contaminating insoluble solid matter from the said viscosityreduced tar, and thereafter removing the .dissolved gaseous hydrocarbons from the tar.

3. The process of claim 2 which comprises subjecting said pressure tar to a flash distillation and removing a distillate fraction therefrom prior to the steps of cooling and separating said tar.

4. In a process wherein hydrocarbon oil is subjected to conversion conditions of temperature and pressure to produce motor fuel, normally gaseous hydrocarbons substantially free from gasoline,'and a relatively viscous Pressure tar which is characterized by having asphalt in solution and by having suspended contaminating iii-.- soluble solid matter therein which renders said tar difficult to filter, the improvement .in refining said pressure tar to make it suitable for fuel purposes which comprises cooling said tar, conducts in'g said cooled tar-into an absorption chamber, a

conducting said normally gaseous hydrocarbons to said chamber, intimately contacting said cooled tar and said normally gaseous hydrocarbons and dissolving about one liquid volume of said normal- 1y gaseous hydrocarbons in each ten liquid volumes of said tar, thus decreasing the viscosity of said tar without precipitating any substantial amount of asphalt therefrom, separating undissolved gaseous hydrocarbons from the viscosityreduced tar, passing the said tar and dissolved normally gaseous hydrocarbons into a separating zone and filtering the said viscosity-reduced ta'r.

to remove contaminating insoluble solid inatter therefrom without removing any substantial amount of asphalt from the tar, and thereafter removing the dissolved gaseous hydrocarbons from" the tar whereby the-tar is converted into afuel relatively rich .in asphalt and relatively free from suspended insoluble, and non-combustible matter. 2

HAROLD V'. ATWELL. 

